1. Field of the Invention
The invention relates to a separator passage structure of a fuel cell, particularly a proton-exchange membrane fuel cell.
2. Description of the Related Art
A unit cell of a proton-exchange membrane fuel cell is composed of a stack body formed by stacking a membrane-electrode assembly (hereinafter, referred to as MEA) and a separator. The MEA includes an electrolyte membrane composed of an ion-exchange membrane, an electrode (anode, fuel electrode) composed of a catalytic layer disposed on one surface of the electrolyte membrane, and another electrode (cathode, air electrode) composed of another catalytic layer disposed on the other surface of the electrolyte membrane. A diffusion layer is provided between the MEA and the separator. In the separator, passages through which a fuel gas (hydrogen) and an oxidizing gas (oxygen, generally air) are supplied to the anode and the cathode, and a passage through which a refrigerant (generally a coolant) passes are provided. A module includes at least one unit cell. A cell stack body is formed by stacking the modules. A terminal, an insulator, and an end plate are disposed at each of both ends of the cell stack body in a direction in which the cells are stacked (hereinafter, referred to as “cell stacked direction”). The cell stack body is fastened by using a fastening member which is disposed outside the cell stack body and which extends in the cell stacked direction (e.g., a tension plate), whereby a fuel cell stack is formed. In the proton-exchange membrane fuel cell, the reaction which decomposes hydrogen into a hydrogen ion and an electron occurs on the anode side, and the hydrogen ion moves to the cathode side through the electrolyte membrane. The reaction which produces water from oxygen, the hydrogen ion, and the electron (the electron generated on the anode side of the adjacent MEA moves to the cathode side through the separator) occurs on the cathode side.Anode side: H2→2H++2e−Cathode side: 2H++2e−+(½)O2→H2O
A concave groove and a convex rib are formed in the separator. The concave groove on a surface of the separator which faces the MEA constitutes a gas passage through which the fuel gas or the oxidizing gas passes. The convex rib contacts the diffusion layer, and constitutes a conductive passage. When a metal separator is used as the separator, the concave groove and the convex rib are generally formed by press molding. The rear surface of the convex rib (i.e., the surface opposite to the surface facing the MEA) constitutes a refrigerant passage. Japanese Patent Laid-Open Publication No. 2001-196079 discloses a separator passage structure of a fuel cell, in which multiple convex portions are regularly disposed on a surface of a metal separator so as to be separated from each other, and the gas flows between the convex portions, that is, a divided convex portion structure.
However, there are the following problems concerning this separator passage structure of a fuel cell. i) It is difficult to make both the gas and the refrigerant flow smoothly. ii) It is difficult to reduce the size of the separator in the cell stacked direction, and thus, it is difficult to make the stack small.
The reason why these problems occur, for example, in the case where the metal separator is used will be described. First, the reason why the problem concerning the flow of the gas and the flow of the refrigerant occurs will be described. Flooding may occur in a part of a surface of the separator, and the gas passage may be blocked due to the flooding. Therefore, it is preferable that a gas cross groove should be formed on each of the convex ribs between the gas passages, and the convex rib should be divided in a direction in which the gas passage extends so that the gas can flow from one gas passage to another adjacent gas passage when the one gas passage is blocked. In this case, the depth of the refrigerant passage on the rear surface of the convex rib is reduced, or the refrigerant passage is divided due to formation of the gas cross groove. As a result, there arise a problem concerning the flow of the refrigerant. Thus, it is difficult to allow the gas to flow smoothly using the gas cross groove when flooding occurs, and to allow the refrigerant to flow smoothly in the refrigerant passage on the rear surface of the convex rib at the same time. Next, the reason why the problem concerning the size of the separator in the cell stacked direction occurs will be described. When the depth of the gas cross groove formed on the convex rib is reduced so that the refrigerant passage on the rear surface of the gas cross groove is provided, the thickness of the separator is the sum of the depth of the gas cross groove and the depth of the refrigerant passage on the rear surface of the convex rib. Therefore, when the gas cross groove with a necessary depth and the refrigerant passage with a necessary depth are provided, the size of the separator in the thickness direction, i.e., in the cell stacked direction inevitably increases. Consequently, it is difficult to make the stack small in the cell stacked direction.